320 THE EEGOLITH 



formed to make the black soil layer. A period of subsidence 

 followed, more loess was deposited and the previous condition 

 repeated, this process going on till all the layers were formed. 



The name adol)e is given to a calcareous clay of a gray-brown 

 or yellowish color, very fine-grained and porous, which is suffi- 

 ciently friable to crumble readily in the fingers, and yet, like 

 loess, has sufficient coherency to stand for many years in the 

 form of vertical escarpments, without appreciable talus slopes. 

 The material of the adobe is derived from the waste of the 

 surrounding mountain slopes, the disintegration being largely 

 mechanical. According to Professor I. C. Russell,^ from whose 

 descriptions is drawn a portion of what is given here, it is as- 

 sorted and spread out over the valley bottom by the action of 

 ephemeral streams, where it becomes mixed with dust blown 

 by the winds from the neighboring mountains, and rendered more 

 or less coherent by the cementing action of interstitial carbonate 

 of lime. 



Hilgard^ limits the name adole to the distinctly clayey soils of 

 the arid regions, and divides them into two classes, — the upland 

 and the valley adobes, the first being derived mainly from the 

 disintegration, in place, of clay shales, while the second are 

 mostly paludal or swamp formations, and represent either the 

 finest materials that remain suspended in slack water, from any 

 source, or sometimes the direct washings of the clayey soils of 

 the hills. Whichever authority we follow, it is evident the 

 name includes materials alike not in mode of origin or com- 

 position, but only in physical characteristics. 



Adobe forms the soil of a large portion of the rainless region 

 of the United States. It is found therefore in Colorado, Utah, 

 Nevada, southern California, Arizona, New Mexico, and west- 

 ern Texas, as well as in the southern portion of Idaho, Wyoming, 

 and Oregon. It has also a wide distribution in Mexico. In 

 the United States it occurs from near the sea-level in Arizona, 

 and even below it in southern California up to an elevation of 

 at least 6000 or 8000 feet along the eastern border of the Rocky 

 Mountains, and in the elevated valleys of New Mexico, Colorado, 

 and Wyoming. 



The maximum thickness of the various deposits grouped 

 under this name is not in all cases readily determined, for the 



^ Subaerial Deposits of North America, Geol. Mag., August, 1889. 

 2 Bun. 3, IT, S, Weather Bureau, Bept. of Agriculture, 1892. 



